In web printing and drying operations, it is often desirable that the web be contactlessly supported in order to avoid damage to the web itself or to the coating (such as ink) previously applied to one or more surfaces of the web. One conventional arrangement for contactlessly supporting the web includes horizontal upper and lower arrays of air bars between which the web floatingly travels. Hot air issuing from the air bars both dries and supports the web. Such dryers for running webs and the like are typically designed with upper and lower hoods. Each array of airs bars is in air-receiving communication with upper and lower headers. The upper header and upper air bar assembly comprise the upper hood of the dryer, and the lower header and lower air bar assembly comprise the lower hood of the dryer. These upper and lower hoods are retractable with respect to each other, primarily to allow a web to be threaded through the machine, and also to allow for the periodic maintenance and replacement of the air bars and other dryer internals.
The upper and lower hood retraction system generally utilizes fluid cylinders to retract the hoods, the fluid cylinders being driven either pneumatically or hydraulically. However, sudden inadvertent pressure loss or the like to the cylinders can cause the retraction system to fail, and the dryer hood to collapse. In order to avoid such a catastrophe, the retracted hood is usually manually blocked in place or automatically pinned in place using auxiliary equipment. However, such a safety measure is generally inefficient and unreliable.